**Core Concept**
Deep sleep, also known as slow-wave sleep, is a stage of non-rapid eye movement (NREM) sleep characterized by high-amplitude delta waves in the electroencephalogram (EEG). This stage is critical for physical recovery, muscle repair, and memory consolidation.

**Why the Correct Answer is Right**
Deep sleep is marked by a decrease in body temperature, heart rate, and blood pressure, reflecting a state of reduced physiological activity. The high-amplitude delta waves in the EEG are thought to be generated by the synchronized firing of thalamic neurons, which are responsible for the regulation of consciousness and sleep-wake transitions. The role of delta waves in memory consolidation is also well-documented, with research suggesting that they play a key role in the transfer of information from short-term to long-term memory.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because deep sleep is not characterized by increased cortical activity, which is more typical of REM sleep.
**Option B:** This option is incorrect because deep sleep is not marked by a decrease in muscle tone, which is more typical of REM sleep.

**Clinical Pearl / High-Yield Fact**
One key clinical pearl to remember is that deep sleep is often disrupted in patients with sleep disorders such as insomnia, sleep apnea, and restless leg syndrome, which can have significant consequences for physical and mental health.

**Correct Answer: C. High-amplitude delta waves in the EEG.**